1. Field of the Invention
The field of the invention is that of compressing fixed images and moving images. To be more precise, the invention concerns optimizing compression in accordance with one or more criteria chosen by the user.
2. Description of the Prior Art
Image compression is necessary in a very large number of fields, in particular with the aim of obtaining small files and/or files that can be transmitted quickly.
The fast transmission of images, in particular, is becoming essential in many fields of application and the image will assume an ever more dominant place in telecommunications.
All multimedia applications, in particular, require fast transmission of images and high-performance image storage techniques. This imperative is encountered, for example, in fields concerning videophones, video-conferencing, electronic mail, telediagnosis, interactive television (VOD: "Video On Demand" and IVOD: "Interactive VOD") up to televirtual reality. The same applies to remote sensing satellites: the increasing resolution of the instruments means that they have to transmit ever greater quantities of image data.
In all cases new image compression techniques are necessary for improved image storage and transmission, depending on the bit rates available, depending on the transmission speed required given the transmission time required and the degree to which information is lost on transmission that can be tolerated.
The standard compression techniques developed over the last five years (JPEG, MPEG) are mainly based on DCT (Discrete Cosine Transform). The compression ratio is generally less than 20 for fixed images and less than 80 for moving images. The increasing importance of the image in telecommunications requires the use of ever more intelligent compression techniques offering ever higher performance. Image compression is therefore one of the crucial features of future telecommunication tools.
The invention concerns such developments and applies in particular to development of the standard JPEG and MPEG techniques mentioned above.
There are many compression algorithms. They can be classified into a number of major families, respectively based on DCT, wavelet transformation, IFS fractals, vector quantification and quad-trees.
Each of these algorithms has its own advantages and disadvantages, depending on the type of image concerned, the required compression ratio and/or the required quality of image reconstruction. On the other hand, none of them can guarantee optimal results under all conditions.
The ideal would therefore be to be able to use all of these algorithms and to choose the most effective algorithm, depending on stated objectives, in each individual case.
However, an a priori knowledge of the most effective algorithm for given conditions and a given image is virtually impossible for a user, especially one who is not an image processing specialist. The problem is made worse by the fact that most of these algorithms necessitate adjustment of one or more parameters.
It is no more feasible to try all possible configurations (choice of algorithm+choice of parameters) for each image to be transmitted or stored.
Prior art compression systems therefore use a single type of compression algorithm. They nevertheless remain somewhat difficult to use because it is necessary to define one or more parameters.
What is more, the various systems operate on complete images but the inventors have found that in many situations only part of the image is of major interest.
One objective of the invention is to alleviate the various drawbacks of the prior art.
To be more precise, one objective of the invention is to provide a method of optimizing the compression of images and/or sequences of images capable of optimizing one or more criteria, or compression objectives, without necessitating from a user any analysis or any a priori choice.
In other words, the objective of the invention is to provide a method of the above kind that is accessible to any type of user, and in particular to users who are not image processing specialists, and which guarantees optimized compression regardless of the type of image and in accordance with objectives set by the user.
Another objective of the invention is to provide a method of the above kind that can assure optimized compression of a very large number of types of images, whatever the objectives set by the user (image quality or compression ratio).